The present invention relates to wireless communications. More specifically, the present invention is related to third generation (3G) cellular systems employing time-division duplex (TDD) for separating base-to-mobile and mobile-to-base communications.
Wireless time-division cellular systems generally divide the time axis into intervals of equal durations called frames. Systems employing the TDD scheme divide time frames into a finite number (NT) of intervals of equal durations, called slots, and allow a cell to use some or all of the slots for uplink (mobile-to-base) or downlink (base-to-mobile) transmissions. The slot assignment of a cell defines how each slot is used by this cell. There are three possible ways for a cell to use a slot: 1) uplink transmissions; 2) downlink transmissions; or 3) the slot is not used at all.
The slot assignment of a cell can be varied by the system in order to adapt to the requirements of the traffic. For example, the system may modify the assignment of one slot from uplink to downlink if the amount of downlink traffic increases while the uplink traffic decreases. In addition, different cells of a system generally do not need to have the same slot assignment. If traffic characteristics in one geographical area are different from another area, the cells covering those areas may have different assignment so as to best adapt to local traffic conditions.
The timeslot assignment Ac of a cell c is represented by a set of NT values, where the sth value of the timeslot assignment (Ac,s), represents the usage of the sth slot in this cell. The number of slots used for uplink and downlink transmissions are denoted Ncu and Ncd, respectively.
FIG. 1 illustrates conflicting slot assignments for two cells in the same vicinity. A first cell 10 has a first time frame 12 which includes a plurality of timeslots NA1 through NAT. The time slots are used for uplink or downlink communications or not all. For the present example, assume that time slot NA3 is used for uplink communications between a mobile unit 18 and a base station 11. A second cell 14, including a second base station 20, is in close proximity to the first cell 10. This cell communicates using a second time frame 15 including a plurality of timeslots NB1-NBT. In the second cell, the slots are also used for uplink or downlink communications or not all. For the present example, assume that timeslot NB3 is used for downlink communications. Because of the cell""s proximity to each other, there is a strong possibility of the second cell 14 causing interference with the communications between the base station 11 of the first cell 10 and mobile unit 18, which leads to system degradation (base-to-base interference scenario). Depending on the degree of isolation, in terms of path loss between the cells, this degradation may or may not be acceptable. The first cell 10 may have to assign the mobile unit 18 to another slot and mark this slot as unusable as an uplink slot in its cell, which reduces the capacity of the system.
Mobile-to-mobile interference scenarios may also occur due to uplink and downlink slot allocations for mobiles that are in close proximity. However, mobile-to-mobile interference is much more upredictable than base-to-base interference and may be mitigated by means of an escape mechanism which reallocates a user""s codes to another timeslot where the interference is less severe.
Therefore, it is important to determine the best slot assignments for every cell, taking into account the conflicting requirements of adapting to local traffic variations and avoiding interference due to different time slot assignments between neighboring cells. xe2x80x9cCrossed slotsxe2x80x9d occur when neighboring cells unconnectedly utilize the opposite slot assignments. A first cell may use the slot assignment for uplink communications and another cell uses the same slot assignment for downlink communications. This results in a possibility that the downlink transmission of one cell will interfere with the uplink reception of another cell. Therefore, it would be desirable to have a system which takes into account the time slot assignment of neighboring cells and efficiently coordinates time slot assignments to increase the overall performance and operation of each cell.
The present invention is a system and method to optimize the number of uplink and downlink slots, given the maximum number of crossed slots between any two cells. The present invention determines the maximum number of crossed slots between any two cells and effectively assigns a direction, either uplink or downlink, to every slot in every cell of a system, taking into account the trade-off between a) avoiding base-to-base or mobile-to-mobile interference; and b) matching the slot assignment of every cell as closely as possible to the local traffic conditions. The present invention assigns users to slots according to their transmission power requirements in order to allow conflicting slot-to-cell assignments between two cells in the same geographic region.